There's some cool functionality they're pitching with streaming to youtube integrated and "join" buttons from the stream, so you can hop right in with someone you're watching (if invited).
I'm a little meh on the whole controller thing instead of kb/mouse -- I can't envision anyone remotely skilled at gaming opting for a controller over kb/mouse. Obviously there's a massive market for consoles, but then again, there's a massive number of horrible gamers out there too.
Regardless, does Google have the computing power to render 4k/60fps for potentially millions of gamers simultaneously? Maybe. Can they do that with minimal (<100 ping) latency consistently? I sincerely doubt it. My skepticism level on that front is at 99%. And if they can't do that, it's just like every other failed game streaming service.